Oil shale represents a vast untapped resource of liquid hydrocarbon fuel; potentially, it could be a source for a synthetic fuel substitute to replace the ever-diminishing supply of domestic petroleum. There are over 2 trillion barrels of synthetic fuels locked up in oil shale deposits in the United States. Oil shale occurs in zones up to several hundred feet thick but typically the richest zone is from 50-100 feet thick. The oil shale is found in the rock in the form of kerogen, a high molecular weight hydrocarbonaceaous material. Heating oil shale in a device termed a reort results in the decomposition of the kerogen and formation of the liquid known as shale oil. Oil shale retorting can be carried out in situ, that is in place in the ground, or it can be carried out in a process facility on the surface. In order to carry out retorting on the surface, it is necessary to mine the ore. The most commonly proposed technique to carry out the mining of oil shale has been the room and pillar method as illustrated in FIG. 1.
The room and pillar method is discussed in Chapter 5 of "An Assessment of Oil Shale Technologies", by the Office of Technology Assessment (June 1980) and in "Oil Shale Mining--Present and Future" by R. B. Crookston and D. A. Weiss taken from "Symposium Papers, Synthetic Fuels from Oil Shale II", Nashville Tenn. Oct. 26-29, 1981, Institute of Gas Technology, p. 417.
In the room and pillar design, the deposit is blocked into mining panels having a panel entry area 10 which may range from 900 to 1500 feet wide. Rooms 14 which are approximately 60 feet wide, the depth of a panel and the height of the mining zone are mined between the rooms leaving a pattern of square pillars 18 from 65 feet to 90 feet on a side to support the roof. Barrier pillars 19 are 65 feet in width but 400 feet in length to separate the panels to assure any pillar failure within a panel will not progress beyond the limits of the panel. Given dimensions vary with the quality of the rock and the depth of the overburden.
The panels are mined in three separate and distinct operations: heading extraction, bench extraction and crosscut extraction. Within the panel the upper extraction, termed the heading, about one-third to one-half of the height of the mining zone is driven within the boundaries of rooms and just below the roof (the top of the mining zone). When the headings are sufficiently advanced, the floor of the heading, i.e., the lower portion of the mining zone is mined. This operations called benching, ranges in depth but averages from one-half to two-thirds of the mining horizon. Benching is advanced in the same direction as the heading. While some crosscuts are taken during the heading advance, most are executed after the benching operation is completed.
The mining consists of several tasks; namely, drilling the blastholes, charging blastholes with explosives, blasting, water application to suppress dust, scaling down the loose rock, loading out the broken rock into trucks and hauling it to the crusher, and supporting the roof with steel rock bolts.
The broken rock in the dump trucks is hauled to a stationary crusher (not shown) located near a slope conveyor in panel entry 10. The shale is crushed and conveyed to the surface via the slope conveyor. Haulage by means of trucks presents a number of safety and logistical problems. The trucks exiting from the panel must traverse a ramp 12 from the heading or bench entries to panel entry 10. Because of the steep grade on ramp 12, the trucks will be subjected to substantial wear. In addition, the steep and narrow ramps 12 will tend to cause frequent accidents. Further, a significant logistical problem can occur at the crusher if the trucks do not arrive on schedule. Trucks off schedule could cause queuing of the rocks at the crushers, resulting in loss of efficiency and productivity.
The heading and benching operations produce oil shale of substantially different grade. In order to maintain constant grade of ore to the retort, the ore must be delivered to the crusher on a preset schedule. For example, in the case of the room and pillar mine discussed above, for every truck from the heading area one or two trucks should come from the bench mining area. In view of the numerous possibilities for delay in arrival of the truck e.g., mechanical breakdown, accidents, unavailability of ore, etc., the use of trucks for haulage of oil can result in frequent failure to maintain a constant grade of oil shale to the retorts. In addition, if the mine is deemed gassy, special requirements relating to mining equipment apply. At present, the large scale trucks required for oil shale haulage are not commercially available for use in gassy mines and would have to be specially fabricated to operate in a gassy mine.
An additional difficulty with a room and pillar mine layout is in the ventilatio requirements. Ventilation may be accomplished by drawing air in one side of the room and pillar design and exhausting out the other. Because of the numerous crosscuts present, proper ventilation becomes virtually impossible. As a result, it is necessary to use numerous curtains or brattices (not shown) to direct the air into rooms where the mining is taking place and close off those areas not undergoing mining. Because the proper seal of the opening is difficult to achieve, these brattices have a tendency to leak. Therefore, while proper ventiltion can thereby be achieved, a substantial amount of power still must be used. The presence of numerous crosscuts results in the need to utilize substantial quantities of power to maintain proper airflow.
Support of the roof of underground mines is an important and expensive procedure. To prevent collapse of portions of the roof, the room and pillar must be designed to withstand the pressure of the rock above and the suspended weight of the roof over the room opening. At an intersection 20 of a room 14 and a crosscut 16, this problem becomes more substantial due to the locally large spans produced by the intersections of crosscut and room tends to be weaker and requires additional maintenance. Minimizing crosscuts would reduce these maintenance costs and decrease the safety risks.